Building blocks of integrated radio Receivers (RXs), for example, Radio-Frequency (RF) parts, Analog Baseband circuit (ABB) and Analog-Digital Converters (ADCs), have limited dynamic ranges. An input power of a wanted signal may vary between, for example, −100 dBm and −15 dBm depending on different wireless or cellular systems. In addition, the power of interfering signals also varies significantly. A radio receiver chain is not allowed to be compressed in any situation by the wanted signal or the interfering and/or blocking signals. In addition, the level of the wanted signal at the inputs of the ADCs must be in a correct range to enable signal sampling and quantization or digitization with a sufficient Signal-to-Noise ratio (SNR) without compressing the ADCs. The dynamic range of the ADCs is not sufficient to cover a variation of approximately 90 dB in the possible input powers of the wanted signal while producing a sufficient SNR. Therefore, adjustable or programmable gain must be implemented in the preceding RF and analog parts to extend the dynamic range of the radio receiver by keeping the level of the wanted signal at the inputs of the ADCs within an optimal range. In modern radio receivers, gain adjustment is usually implemented with digital control, i.e. the RX gain can have different values having a certain difference, i.e. gain step, between them. The difference between consecutive gain values or the gain step may be, for example, 3 dB or 6 dB. The value of 6 dB is used quite commonly.
The accuracy of gain steps in programmable gain in a primary or diversity RX must meet certain requirements. The power of a received signal is measured in the RX of a Mobile Station (MS) and communicated to a Base Station (BS) or cellular network so that the BS can transmit data with an output power, which is optimal for the network capacity. The power of the received signal is usually measured in the digital circuit of the RX after the programmable analog gain implemented in the analog parts of the RX. If there is an error in the measured signal power due to an error in a gain step in the analog parts of the RX, the output power of the BS Transmitter (TX) for the corresponding MS is not optimal. This can affect the capacity of the BS or cellular network. Therefore, the required accuracy of power measurement in the RX is specified, which also sets requirements for the accuracy of the gain steps. Gain steps having different values or sizes, for example, multiples of 6 dB, may have different specifications.
Different gain values in an RX can be calibrated to correct values for each sample in production. Unfortunately, since there may be several gain settings in a single RX and there can also be several RXs integrated on the same chip, lots of parameters need to be measured and calibrated. In addition, one RX can have multiple Low Noise Amplifiers (LNAs) operating in different RF bands. This kind of calibration in production is expensive and should be avoided or minimized, if possible. A Measurement radio Receiver (MRX) is usually used to measure or monitor the TX output signal power and quality to provide data for calibration and tuning of the TX to optimize its performance. The MRX is usually integrated on the same chip with the rest of transceiver. Since the MRX is used, for example, to measure and calibrate the output power of the TX, the accuracy requirements for relative gains and, therefore, also for gain steps, in the MRX are stringent. Again, the amount of calibration of the MRX in production should also be minimized.
In modern radio RXs, most of or all of Direct Current (DC) offset compensations are usually implemented in digital circuits. If programmable gain is implemented at baseband in an RX and baseband gain is changed, the output DC offset may change abruptly, which may lead to degraded output signal quality. The digital DC offset compensation may not be able to react fast enough to a sudden change in the output DC offset. A change in the output DC offset due to a change in baseband gain can lead to degraded signal quality even if the change does not occur during the actual reception, for example, when the digital DC offset compensation subtracts an incorrect DC value from the digital output signal leaving part of the DC offset uncompensated. Such issues are possible in the primary and diversity RXs, as well as in the MRX. Therefore, implementation of programmable gain at baseband is not usually feasible.
Due to large interfering or wanted signals, the RX gain preceding the ABB needs to be decreased to avoid compression in the ABB. Thus, part of the programmable RX gain is usually implemented in the LNA, but it is challenging or technically not feasible to implement all of the programmable gain in the LNA. Since the maximum voltage gains of LNAs are usually 20-30 dB, the implementation of all required programmable gain in the LNA, for example 40-50 dB, would require the LNA to operate as an attenuator in lower gain settings, which would significantly increase the Noise Figure (NF) of the receiver and lead to a solution that would not be feasible. Since it is not desirable to implement programmable gain in the ABB and it is challenging to implement all programmable gain in the LNA, the rest of the programmable gain has to be implemented in down-conversion mixers.
Passive current-mode mixers are widely used in radio RXs because of their high dynamic range. Programmable gain may be implemented in the passive current-mode mixers by using a switched transconductance stage, gm-stage, comprising parallel branches that can be switched on and off. The problem with this solution is lower linearity and dynamic range because of the unavoidable nonlinearity of the voltage to current conversion in the gm-stage. The LNA voltage gain, which may be 20-30 dB, precedes the gm-stage of the mixers making the linearity requirements of the gm-stage stringent. Increasing the supply current to improve the linearity of the gm-stage is not a feasible solution. The lower linearity makes this solution less attractive.
Programmable gain may also be implemented in such passive current-mode down-conversion mixers by using programmable switched resistors in series with the mixing transistors in the mixers. One possibility to implement a switched resistor network, as described in A. S. Sedra, K. C. Smith, Microelectronic Circuits, Saunders College Publishing, USA, 3rd edition, 1991, pp. 744-745, is to use an R2R network that can realize different gain levels with a 6-dB gain step, as shown in FIG. 1 (a). The R2R network is a resistor network that comprises cascaded repeating branches of a series resistor R and a switched shunt resistor 2R. However, in the passive current-mode mixer, the mixer switching devices have a non-zero parasitic input resistance, which degrades the accuracy of the gain steps in such configuration. The accuracy of the gain steps is limited in practice and can be insufficient in some applications, especially in an MRX but also in primary and diversity RXs. Another problem with the R2R network is that it cannot guarantee a fixed or well-regulated load impedance for the LNA.